This invention relates to a partially coated optic fibre and in particular to a partially coated optical fibre for laser transmission which may be incorporated into optical devices or telecommunication systems.
Since their inception in the mid-1960's, the growth of the use of optical fibres in optical devices and telecommunication systems has been substantial. This growth has been due largely to the remarkable data carrying capacity displayed by fibres.
However, optical fibres do have a significant draw back, in that they are relatively brittle and fragile. Therefore, optical fibres are commonly provided with a protective coating of a plastics material, commonly a UV curable material such as an acrylate or a UV curable polymide. Desolite 950-106, 950-108, 950-131 and 3471-3-14 are routinely used for coating optical fibres for general purpose uses. Thermally cured silicon coatings, such as Sylgard 180 and 184 are also available. However, when these materials provide a relatively soft and tacky coating which is used in either high temperatures or chemical environments in which acrylate coatings are not suitable.
For practical use in telecommunication systems such fibres may be combined into multi-fibre cables formed, for example, from a polyurethane outer jacket. If additional strengthening is required the jacket may be provided with strengthening members formed, for example, from Kevlar. However, as this application relates to the cladding of individual fibres such multi-fibre cables will not be discussed further herein.
In use fibres tend to be end coupled and end pumped, at least in part because of the requirement to coat the fibres. That is, any useful radiation within the fibre is injected through the end face of the fibre, travels the full length of the fibre through internal reflection, in the normal manner, and is transmitted from the remote end of the fibre.
However, in some situations there are significant advantages to be gained from access to a circumferential portion of the fibre. By far the most important of these is the use of a light source to produce permanent light sensitive changes within the optical fibres. In other words advantages are gained from the use of a light source to produce a periodic refractive index modulation within the fibre in the form of a grating. The most common light source used is a UV laser such as an Eximer or Argon ion laser. Such gratings can be used in a host of devices from sensors and band pass filters to fibre laser mirrors.
Applications which require the removal of the coating over lengths exceeding 50 mm include, for example, fibre grating dispersion equalisers and long period gratings which may be written into the core or cladding for applications which include gain flattening, polarisation rocking and or loss filters, wave length selective polarisers and cladding mode pumped amplifiers. For example, a dispersion equaliser requires up to eight meters in length of coating to be removed from the fibre, which leaves a substantial length of fibre which is open to contamination or physical damage.
In this device a long chirped grating is written into a stripped fibre spanning the 32 nm gain window of an erbium amplifier. Such a device, when connected to the first output port of a three port circulator provides equalisation of dispersed signals transmitted down the fibre. The chirped grating simply removes the wavelength dependent time delay spreading arising from the dispersion in the transmission fibre.
Long period grating applications operate on the basis that a refractive index variation along the length of a fibre has a period that matches the difference in propagation constants for the lowest order mode in the fibre core and some higher order mode in the cladding. This causes power to be transferred from one mode to the other, which can be used to provide wavelength selective loss and polarisation rocking in two moded fibres. In cladding mode pumped amplifiers by writing the index changes in the cladding power can be transferred from the cladding to the core more efficiently than would otherwise be possible.
A further area in which the invention may be useful is in the area of non-intrusive taps wherein a loss mechanism such as microbending can be used to eject light from the length of the fibre. If the coating is removed this light can more easily and efficiently be accessed. Therefore, there is a conflict between the need for access to the fibre and the need for protective coatings.